In a large bowl, add the pork, herbs, fish sauce, garlic, red pepper flakes (if using), and salt. Use your hands to thoroughly mix everything together. Form into 24 small meatballs (use a tablespoon to measure each ball).
In a large skillet over medium-high heat, warm the oil. Once the oil is shimmering and hot, add in the meatballs, spacing them out evenly in the pan. Let cook for 5 minutes, or until deeply browned. Reduce the heat to medium-low, flip each meatball, and let cook for 5-7 more minutes or until cooked through, giving the pan a shake halfway through. Covering the pan while cooking can help speed this along. If browning or burning, reduce your heat to low. Transfer to a paper towel lined plate.
In a medium bowl, add the carrot, cucumber, vinegar, water, and salt. Stir together and press to submerge in the liquid (it won’t all be submerged, so don't worry). Let sit for at least 10 minutes, but longer is better (overnight is best, so make ahead!).
In a small bowl, mix together the dressing.
Place the meatballs into each lettuce or cabbage cup. Add the avocado, pickled veggies, and herbs. Drizzle with the dressing (you only need a little bit).